Did the kids play thier own instruments? It depends on the kid in question. For the most part the main kids did not play thier own instruments. The dancers did not play instruments in real life.
Why were there no episodes in 1990? It was never explained why 1990 featured no new shows. Where the series returned in 1991, only Kenny Ford and Jennifer Love Hewitt returned. Many of the other cast members left for other projects. Cory Tyler was on NBC's A Different World, a role he had in 1989 and 1990.
Were the songs changed for the song? While there were original songs for the series, the show used songs by famous artists. Some songs had to be edited for content such as sexual references. In season six, a song by After 7 had a line about sex removed.